The invention relates to polyurethane foams and, more specifically, to processes for making polyurethane flexible foams using silicone-based surfactants as cell stabilizers. In particular, processes for making polyurethane foams using a silanol-based surfactant system as a stabilizer for the foam are disclosed.
Polyurethane foams and their preparations are well known in the art, having applications in a wide variety of areas. Typically, polyurethane (PU) foams are produced by reacting a polyisocyanate with compounds containing two or more active hydrogens, generally in the presence of blowing agent(s), catalysts, silicone-based surfactants and other auxiliary agents. The active hydrogen-containing compounds are typically polyols, primary and secondary polyamines, and water. Two major reactions are promoted by the catalysts among the reactants during the preparation of polyurethane foam, gelling and blowing. These reactions must proceed simultaneously and at a competitively balanced rate during the process in order to yield polyurethane foam with desired physical characteristics.
Reaction between the isocyanate and the polyol or polyamine, usually referred to as the gel reaction, leads to the formation of a polymer of high molecular weight. This reaction is predominant in foams blown exclusively with low boiling point organic compounds. The progress of this reaction increases the viscosity of the mixture and generally contributes to crosslink formation with polyfunctional polyols. The second major reaction occurs between isocyanate and water. This reaction adds to urethane polymer growth, and is important for producing carbon dioxide gas which promotes foaming. As a result, this reaction often is referred to as the blow reaction. The blow reaction is essential for avoiding or reducing the use of auxiliary blowing agents.
A superior quality flexible molded foam displays several important characteristics. It will have good bulk, vent, and shear stability which implies the foam has a small, uniform cellular structure throughout the interior of the foam. These foams will also display good surface stability, defined as having a layer of fine cells adjacent to the outer surface of the foam, and good dimensional stability (i.e., exhibit a reduced tendency to shrink after being removed from the mold). Foams that are less susceptible to shrinkage will be easier to process, require less mechanical crushing which can weaken the physical integrity of the polyurethane, and have lower scrap and repair rates. Superior quality non-molded flexible foams primarily require good bulk dimensional stability, which if absent will lead to foam collapse or severe densification. Reducing the overall emission of additives from a flexible foam [Volatile Organic Compounds, “VOCs”] is also desirable, particularly in car interior applications where automotive windshield fogging can be a problem. For example, one of the main components of VOCs evaporating from flexible molded foams is the amine catalyst.
The manufacturing equipment and chemicals have an important effect on the quality of the foam; however, the surfactant is often one of the most critical components of the formulation as it has a direct and significant influence on the bulk, vent, shear, surface, and dimensional stability as well as the emissions of the foam. In the past, chemical strategies for selecting formulation variables in order to optimize bulk, shear, vent, surface, and dimensional stability have been successful for many polyurethane foam applications. Key variables include the judicious selection of surfactants and catalysts, and the incorporation of cell opening polyols.
The foam industry is now facing cost reduction issues, and is challenged with maintaining foam physical properties while at the same time reducing their raw materials and processing costs. Approaches have included reducing foam density by incorporating more water in the formulation or injecting liquid carbon dioxide, lowering the amount of relatively expensive graft copolymers, using blends of TDI/MDI, and incorporating isocyanate terminated prepolymers. All of these approaches have placed increasing challenges on the accompanying additives, particularly in terms of maintaining foam dimensional stability.
The surfactant composition utilized in the polyurethane foam is often one of the most critical components of the formulation as it has a direct and significant influence on the overall dimensional stability as well as the volatile emissions of the foam. One such strategy to provide a foam having open cells is to employ a silicone-based surfactant, such as polydimethylsiloxane (PDMS) fluids and/or organomodified PDMS fluids to stabilize the foam until the product-forming chemical reaction is sufficiently complete so that the foam is self-supporting and does not suffer objectionable collapse. Additionally, the silicone surfactant should help provide open foam at the end of the foaming process, this being particularly critical when producing HR foams. Examples of such silicone-based surfactants are short polydimethylsiloxane surfactants having from about two to about seven siloxane units. This type of surfactant is generally of low molecular weight and mobile thus stabilizing the foam without closing the cell structure. A drawback associated with the use of this type of surfactant is that when forming components such as, for example, foam seats, headliners, sun visors, etc., employing a polyurethane foam based on this type of surfactant, the unreacted low molecular weight surfactant will volatize from the polyurethane foam and subsequently deposit on, for example, the car windows, as an oily film. This, in turn, scatters light resulting in poor lighting conditions for the driver. It would therefore be desirable to employ a siloxane-based surfactant which provides adequate bulk, vent, surface and dimensional stability to polyurethane foam systems but is also retained within the foam at elevated temperatures, thereby producing polyurethane foams having reduced VOC emissions while imparting excellent physical properties without substantially closing (tightening) the cells of the polyurethane foam.
Accordingly, a number of varied approaches to the development and use of alternative silicone surfactants in polyurethane foam production have been advanced. In U.S. Pat. Nos. 6,245,824; 6,235,804 and 4,797,501 the use of siloxane-oxyalkylene copolymers (silicone polyether) surfactants as foam stabilizers for the preparation of polyether based flexible polyurethane foam is illustrated. These patents suggest the use of silicone copolymers functionalized with polyethers as the active compounds in polyurethane surfactant blends.
Nelson, et al, “Silicone Modified Polyurethanes”, Proceedings, The 8th Annual BCC Conference on Flame Retardancy, Stamford, Conn. (1997) reported utilizing difunctional polydimethylsiloxanes bearing carbinol and silanol groups to increase the fire-retardancy of polyurethane elastomers. These difunctional PDMSs differed from the other silicone-based surfactants in several ways. First, the functionalized polydimethylsiloxanes described previously in the art were used as the main component in the formation of polyurethane elastomers. Secondly, the functionalized siloxanes were used to increase the flame retardancy of solid polyurethane elastomer systems.
U.S. Pat. No. 6,239,186 discloses the use of particular siloxane oligomers as foam stabilizers for the production of open cell polyurethane foam. These organosiloxanes were polydimethylsiloxane surfactants with a narrow molecular weight distribution, the use of which resulted in stabilized, open-cell polyurethane (PU) foams.
U.S. Patent Publication 2004/0152796 disclosed the use of hydroxyalkyl- or carbinol terminated siloxanes as surfactants for the production of low emission polyurethane foam. This disclosure focuses on the use of siloxanes bearing carbinol (COH) functionalities as the active surfactant component in the production of low/reduced VOC emission polyurethane foam.
Thus, there still exists a need for highly efficient foam stabilizing compositions comprising low-emission, silanol-containing organosiloxanes, which can be used alone or in combination with other polymeric siloxane fluids, to produce open cell polyurethane foams having reduced VOC emissions.